We will investigate the sequence of ultrastructural and molecular changes in the rat mast cell membrane during histamine secretion stimulated by the calcium inophore, A23187. A selected group of agents previously studied for their effects on mast cell secretion will be utilized to examine the significance of the demonstrable events in the secretory process and to establish a temporal sequence of these events. Agents have been chosen for their ability to affect microtubules, microfilaments, cAMP, ATP, SH-dependent enzymes and membrane structure. Electron microscopy of thin sections and freeze fracture specimens will be used together with phospholipase and fluorescent probes of membrane structure to characterize the secretory events. Studies on mast cell granule structure will be carried out with attention to the association between heparin and mast cell protease. The molecular site of histamine binding will be examined in intact and reconstituted granules.